The present invention relates to connectivity between high-speed trains and network nodes of a cellular communications system.
Operators of cellular communications systems who have invested in deployments of networks along high-speed train corridors have found that the achievable system capacity is significantly limited by the User Equipment (UE) performance. Consequently, strategies for improving the downlink system capacity for communication between network nodes and UEs on high-speed trains have been studied for some time by affected organizations, such as the standardization body known as 3rd Generation Partnership Project (3GPP). Four exemplary network deployments for serving a high-speed train were identified by operators and described in 3GPP TSG RAN Meeting #66, RP-141849, “Motivation of New SI proposal: Performance enhancements for high speed scenario” (Maui, Hi. (US), Dec. 8-12, 2014). These exemplary scenarios are depicted in FIGS. 1A, 1B, 1C, and 1D and described in the following.
FIG. 1A shows “Scenario1”, in which a dedicated network is deployed along the railway. Examples include Remote Radio Head (RRH) (or other antenna node) deployments. Separate carriers, f1 and f2, are utilized for the dedicated and public networks, respectively. By sharing the same Cell Identifier (ID) among multiple RRHs, the handover success rate can be increased to some extent.
FIG. 1B shows “Scenario2”, in which separate carriers are utilized. One carrier, f2, has good coverage and serves as a Primary Cell (PCell) for mobility management (control). Another carrier, f1, at high frequency is deployed along the railway and provides good data transmission. Carrier Aggregation (CA) or Dual Carrier (DC) could be applied.
FIG. 1C shows “Scenario3”, in which a public network is deployed along the railway and repeaters are installed in train carriages. With repeaters, the signal quality can be improved, although the penetration loss is large.
FIG. 1D shows “Scenario4”, in which a dedicated network, using for example frequency f1, is deployed along the railway, and repeaters are installed in train carriages. A public network using another frequency, for example f2, serves other UEs that are not onboard the train.
In addition to the scenarios described above, another scenario involves using Access Points (AP) onboard high-speed trains. The Access Point offers UEs onboard the train to connect via a local network (e.g., 3G/4G pico cells or WiFi) and establishes a wireless backhaul link to the Radio Access Network (RAN). From the point of view of the Radio Access Network (RAN), the AP aggregates multiple users into a single one, with benefits including, for example, reduction of handover-related signaling overhead. Moreover, an AP-based solution allows UEs to save power since the penetration loss through the walls of the carriage can be avoided, thereby allowing the UEs to use lower transmission power.
The current communication standard has partly taken UE speeds up to 300 km/h into account, but only for the data demodulation part; for cell detection only 40 km/h has been considered. With increased deployment of high speed train lines, number of UEs and machine-type communication (MTC) devices, and increased usage of bandwidth per user, dominating operators are requesting improved UE downlink performance for speeds up to and exceeding 350 km/h.
The inventors of the subject matter described herein have considered that, depending on relative movement of a UE to a transmitting site, the received signal may display a significant Doppler shift. The Doppler shift forces the UE to increase its demodulation frequency when moving towards the cell, and decrease it when moving away from the cell, the increases/decreases being relative to the carrier frequency used in the network. The magnitude of the Doppler shift depends on the relative velocity of the UE towards the transmitting antenna. If the transmitting antennas are placed close to the track, the angle between the trajectory of the UE and the line between the UE and the transmitting antennas will be small. Consequently, a substantial part of the UE's velocity will contribute to a Doppler shift. Moreover, there will be an abrupt change of sign of the Doppler shift when the UE passes the transmitting antenna, and the smaller the angle, the more abrupt the change is.
Given an angle, α, between a first vector representing the trajectory of the train (and hence also the UE) in Euclidean three-dimensional space and a second vector (also in Euclidean three-dimensional space) between the train and the transmitting antenna, the Doppler shift can be expressed as
      Δ    ⁢                  ⁢    f    =      f    ⁡          (                                                  1              -                              v                c                                                    1              +                              v                c                                                    -        1            )      where c is the speed of light and ν is the relative velocity of the UE (essentially the same as the velocity of the train) towards the transmitting antenna. With an angle α as discussed above and UE velocity, νUE, the relative velocity towards the transmitting antenna giving rise to Doppler shift is ν=νUE cos α.
The inventors further note that, if an AP-based solution is employed, and assuming that the AP can provide a stable frequency reference, the Doppler shift will only be experienced on the backhaul link and hence not by the UEs. Thus, the Doppler shift can be dealt with by the AP without requiring any special technology in the UEs for this purpose.
There is therefore an advantage to employing an AP-based solution in high-speed train scenarios. However, the inventors have recognized the existence of a number of problems with conventional approaches. The achievable system capacity depends on the performance of the wireless backhaul link between the AP and the RAN. In particular deployment scenarios such as Scenario1, referred to as Bidirectional Single Frequency Network (SFN) deployment, the performance of the backhaul link is degraded by fading and inter-carrier interference caused by receiving the same signal from two directions simultaneously and with opposite Doppler shifts.
Bidirectional SFN has already been deployed along high-speed train corridors by dominant operators, and they have found that the achieved system throughput is not nearly as good as expected. As a consequence, it may take longer time to get a return on investments, and the end-user experience may be bad. It has been proposed in 3GPP RAN4 to define a high-speed enabled UE category having an improved receiver to mitigate the system capacity loss. This will however not lead to improved capacity as long as the bulk of the UEs are legacy terminals. The reason is that those terminals will consume the available capacity by their use of redundancy (robust encoding) and retransmissions.
Since investments on the network infrastructure already have been made, it is important to find ways of mitigating the low downlink performance by technology that allows legacy terminals to perform better. In particular, there is a need for an improved backhaul link for an AP-based solution in the high-speed train environment.